This application claims the priority of Swiss application 3477/95 and PCT/CH96/00420, filed Dec. 8, 1995 and Nov. 27, 1996, respectively, the disclosures of which are expressly incorporated by reference herein.
The present invention is based on problems and requirements which have arisen during the manufacturing of CDs. However, in principle, the resulting solutions can be used for other applications. For this reason, the present description will first specifically start out from the requirements and problems during the manufacturing of CDs and solutions according to the present invention will be described in order to then, in a generalizing manner, indicate how the discovered principles can be used in general.
In the manufacture of CDs, it has been accepted to treat the individual plastic substrates at very short cycle times per processing step, specifically in the second range. In this case, the vacuum sputtering technique has been accepted for applying the reflecting metal layer. The subsequent lacquering with a protective lacquer takes place in a wet coating process.
In particular, the subsequent fast hardening of the lacquers under ultraviolet light presents a problem for the reliability of the CD manufacture.
The above-mentioned lacquering also has no relationship to the vacuum coating technique which is normally used for metal coating.
So that the very short metal coating cycle times are not canceled out by much longer wet lacquer coating process cycle times, high technical expenditures are required for the lacquering process.
It is an object of the present invention to provide a vacuum treatment chamber which permits the implementing of the above-mentioned protective layers in a vacuum process of a type related to the sputtering technique with the required short cycle times.
Methods are definitely known from the vacuum coating technology for depositing non-conductive layers, such as corrosion protection layers. However, normally significantly longer coating times must be accepted than the above-mentioned required few seconds.
If, as, for example, in the case of known so-called plasma enhanced chemical vapor deposition (PECVD) processes with microwave plasma discharges in the interior of the plasma discharge, coating rates of approximately 40 nm/sec are possible, the plasma densities required for this purpose are so high that the resulting temperature stresses do not permit a coating on plastic substrates. For maintaining the plastic-compatible temperatures (for example, of PMMA or polycarbonate), the substrate would have to be moved out so far from the range of the highest plasma density because, as the result of the coating rate which is lower there, the required short cycle times could not be maintained.
Also, according to experiences, coatings which are deposited at a high rate in the marginal range of microwave discharges, frequently have a loose construction and are therefore unsuitable for a use as corrosion blocking layers.
Summarizing, it may therefore be stated that the combined meeting of the short cycle times in the second range with the required coating thickness and the limiting of the temperature-caused stress as well as the maintaining of a sufficient layer quality so far has not been considered possible by using vacuum coating methods.
Of a less basic nature, also known high-frequency CVD processes have the disadvantage that not only the substrate but also HF coupling-in arrangements are coated, whether, in the case of microwave plasmas, these are dielectric coupling-in windows or, at lower frequencies, metallic coupling-in electrodes. The cleaning with the exchange of the mentioned parts or by a plasma-chemical in situ cleaning is not compatible with the requirement of short cycle times to be maintained over long time periods.
It is therefore another object of the present invention to develop a system and a method for a fast and economical depositing of layers from the gas phase, in which case a low loss of coating material and a high homogeneity are required.
In the case of a high-frequency plasma treatment chamber which, with a view to the CD production problems, is then constructed as a coating chamber and in this case, is also designed particularly for a PECVD process, the above-mentioned problems are solved by providing that the high-frequency discharge current circuit includes the substrate as a capacitative coupling-in element.
While, in the case of the specific application to metal-coated, specifically plastic substrates, as in the production of CDs, the substrate cannot be used as a microwave coupling-in window, according to a general aspect of the present invention, the microwave coupling-in by the dielectric substrate is definitely possible if the carried-out coating is also dielectric.
According to the invention, a metal-coated dielectric substrate basically also takes over an electrode function with respect to high-frequency plasmas in the lower frequency range.
It is the basic recognition that, as a result, the required high-frequency output can be kept low which solves the problem of the temperature-caused stress. However, such high coating rates are simultaneously achieved that the required effective protective layers can be deposited within extremely short coating times, of even one second.
In addition, it was found that the thus deposited protective layers, that is, deposited in a high-frequency PECVD coating process by means of the chamber according to the present invention, while the layer thickness is comparable, are even harder than conventional lacquer layers. Furthermore, the layer depositing takes place virtually only on the substrate surface to be coated which acts as the high-frequency coupling-in surface.
More specifically on a PECVD coating chamber, the above-mentioned problems can be eliminated, by its construction which first has the object of minimizing the coating of process chamber walls, other than the substrates to be coated. However, when this solution is considered by itself, it has the disadvantage that the coating rate on process chamber walls which are not covered by the substrate remains high so that, in a longer operation without any cleaning, problems occur as the result of the chipping-off of layers. On the other hand, also in the case of this chamber, high coating rates are achieved at the required low stress temperatures.
A chamber which is optimized in every respect is obtained by the simultaneous implementation of the coupling-in technique of the present invention on the chamber with the minimal volume.
Therefore, a protective coating process for storage disks, particularly optical storage disks, such as CDs, according to the invention is also provided which, in the continuous manufacturing operation is designed as a vacuum coating process and is therefore of the same type as the fast sputtering process normally provided for metal coating. A high-frequency PECVD process is preferably used in the case of which the high-frequency plasma discharge energy is coupled into the process space by way of the substrate.